1. Field of Invention
This invention relates to the detection and monitoring of retinal physiology and pathology, and more particularly to detecting damage to the retina caused by glaucoma.
2. Background of the Invention
Glaucoma is clinically characterized by recognizable patterns of visual field loss and optic nerve head pallor and excavation. Pathologically, there is atrophy of the ganglion cell and nerve-fiber layers of the retina. Glaucoma is the leading cause of irreversible blindness worldwide and the second leading cause in the United States, after macular degeneration.
The most widely accepted and heretofore effective means of diagnosing and monitoring glaucoma is the automated threshold-type visual field test such as performed by the Zeiss-Humphrey Model 750 Field Analyzer (Zeiss-Humphrey, Inc., Dublin, Calif.). Visual field tests are designed to map a person""s visual field and document the level of peripheral vision. The test consists basically of responding every time a flash of light is perceived, all the while looking straight ahead. The test is computer controlled and lasts about five minutes per eye. Unfortunately, patients vary in their attentiveness and response time. Longer field tests are more likely to result in fatigue and diminish the ability of the patient to maintain peak concentration. Furthermore, there are many reasons other than glaucoma for an abnormal visual field result: (1) the test was poorly given due to technician error, (2) the instrument was defective, (3) the patient did not grasp how to take the test, (4) the patient was tired, (5) the defect is accounted for by some pathology other than glaucoma, such as a neurological disorder or retinal disease.
Accordingly, what is needed in the art is a fully objective and reliable technology for the detection and monitoring of the characteristic retinal damage caused by glaucoma.
It is, therefore, to the effective resolution of the aforementioned problems and shortcomings of the prior art that the present invention is directed.
However, in view of the prior art at the time the present invention was made, it was not obvious to those of ordinary skill in the pertinent art how the identified needs could be fulfilled.
The present invention includes a method of detecting nerve fiber layer and ganglion cell damage of the retina, representative of glaucoma, in vivo. It includes the steps of: imaging a retina, establishing a wavelength of light wherein reflections returned from healthy and damaged retina are substantially identical or similar. Baseline values for this wavelength have been empirically established at 500 nm or the spectral region from about 450 nm to about 600 nm. A second wavelength is determined wherein reflections from healthy and damaged retina substantially diverge. Such wavelengths have been empirically observed at 825 nm or the spectral region from about 750 nm to about 875 nm, although additional or specific wavelengths may be suitable. The differences between reflections at the first and second wavelengths are then recorded as a function of location on the retina and are used to provide a map of glaucoma damage. Measuring a plurality of wavelengths known to correlate with healthy and damaged pathology would lead to greater accuracy in an analysis. Such a map can be generated using analog or digital methods. A digital approach will most likely be easier to standardize and made available for widespread commercial use.
In a first embodiment of the invention, developed primarily for research purposes, a photographic slide of a human retina in vivo is obtained. A thermal (e.g., tungsten) light source is preferred for exposure of the photographic slide as flash sources may produce spectral lines. High-speed film (ISO 400) and a relatively low-light source, with an exposure of about one second is employed to obtain the necessary imaging and detail of a human retina in vivo. The slide is then illuminated by transmitted light by a tungsten, zirconium arc, or other thermal source lamp and focused onto a spectrometer. The retina is divided into a two-dimensional grid and each grid point is then plotted for the above-mentioned values. Wavelengths between 500 nm and 600 nm and between 750 nm and 875 nm have been empirically shown to differentiate healthy and damaged retinal layers. The relative difference between the spectral intensity of the first and second wavelengths may then be plotted in three-dimensional relation to the two-dimensional grid. Peaks or valleys on the grid are then indicative of retinal nerve-fiber layer and ganglion-cell layer damage, while a smooth uniform surface on the grid indicates a lack of pathology.
A second embodiment of the invention employs an array of band pass, low pass, or high pass filters to accept only the two wavelength bands of interest from the image of the retina (the baseline value and the variable value). As an in-vivo application, the image of the retina is split in two by an optical beam splitter and filtered. Synchronized CCD or video cameras capture the two images. The filtered images are then detected by the cameras and differenced using a computing means. An advantage of using the filters is that spectroscopic analysis is not needed and only the resulting intensities left after filtration need be examined.
A third embodiment of the invention employs single-wavelength or narrow line width lasers such as diode lasers or dye lasers or Optical-Parametric-Oscillator lasers which can tune to the spectral regions of interest. A laser is provided at the baseline wavelength (e.g. 500 nm) and at a wavelength sensitive to pathology. The reflection intensities of the retina at the wavelength of interest are mapped and analyzed to detect pathology.
An advantage of the present invention is that it provides a diagnostic tool for detecting and monitoring early and advanced visual field loss in glaucoma, and also for investigation of other retinal diseases such as macular degeneration and diabetic retinopathy.
Another advantage of the present invention is its ability to provide a diagnostic tool to examine a patient for suspected glaucoma even if there is no visual field loss.
Another advantage of the present invention is its ability to provide a rapid and objective diagnostic alternative to the traditional visual field test.
Another advantage of the present invention is its ability to provide a diagnostic tool that can differentiate between glaucoma and other disorders that might affect the traditional visual field test.
Another advantage of the present invention is its ability to provide a diagnostic tool that enables the study of low tension glaucoma.
Another advantage of the present invention is its ability to provide a diagnostic tool that produces consistent and reproducible results for long-term monitoring of a patient""s retinal condition.
It is to be understood that both the foregoing general description and the following detailed description are explanatory and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute part of the specification, illustrate embodiments of the present invention and together with the general description, serve to explain principles of the present invention.
These and other important advantages and features of the invention will become clear as this description proceeds.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts that will be exemplified in the description set forth hereinafter and the scope of the invention will be indicated in the claims.